1. Field of the Disclosure
Embodiments disclosed herein relate generally to an improved hydraulic control system for actuation of subsea equipment. More specifically, embodiments disclosed herein relate to apparatus and methods for monitoring the actuation of deepwater subsea blowout preventers (“BOPs”) with a hydraulic control system.
2. Background Art
Deep water drilling for oil and natural gas is conventionally conducted through a subsea blowout preventer (“BOP”) stack, which may be removably attached to a wellhead proximate the seabed. One or more subsea BOPs in the stack may be closed to shut-in the wellbore if, for example, pressurized fluids enter the wellbore from a geological formation. Subsea BOP stacks may be controlled from the surface by one of a number of control system types, such as hydraulic or electro-hydraulic systems, including multiplexed (“MUX”) electro-hydraulic control systems.
The earliest subsea BOP control systems were hydraulic systems, and a large number of them continue to be employed today. Hydraulic systems are generally cheaper and more robust than electro-hydraulic systems. Hydraulic systems, for example, generally have higher up-time than electro-hydraulic systems, are easier to diagnose, require fewer spare parts, and can be repaired in the field by non-specialized workers. Studies have shown that MUX electro-hydraulic BOP control systems may have an initial cost about 4 times that of a hydraulic system and over a 5-year period average about 1.8 times more downtime. Because downtime on a modern floating drilling rig can today cost on the order of $20,000 per hour, the increased downtime of MUX control systems has become a significant issue.
In deep water, however, prior-art hydraulic BOP control systems may experience delays in subsea BOP response time; for this and other reasons, electro-hydraulic control systems, especially MUX systems, may now be typically preferred for drilling in deep water, especially in waters deeper than about 5,000 feet.
Industry standards (such as those of the American Petroleum Institute (“API”) prescribe maximum “closing times” for subsea BOPs, regardless of water depth; typically, annular BOPs are required to close within 60 seconds and ram BOPs are required to close within 45 seconds. Naturally, in the interests of improved safety, it is an industry goal to execute these functions as fast as practically possible.
Closing times are generally defined as the elapsed time from actuating a selected subsea BOP function at the surface (that is, on the drilling vessel) until such point that a return signal from the BOP stack has arrived back at the surface indicating that the selected BOP function has been completed. The process of actuating a subsea BOP function generally comprises 4 discrete steps: (1) sending a signal to the subsea BOP stack from the surface, (2) opening of at least one hydraulic valve on the subsea stack in response to the signal from the surface, (3) hydraulic actuation of the selected BOP function, and finally, (4) sending a signal to the surface that the BOP function has been successfully actuated.
In a prior-art hydraulic control system, indication that a selected BOP function has been successfully actuated may be provided by a pressure gauge at the surface, which is connected by way of an umbilical hose to a hydraulic manifold on the subsea stack, which powers the hydraulic actuation of the selected BOP function. When the selected BOP function is initially actuated, the pressure in the subsea hydraulic manifold drops. When the BOP function has been completely actuated, the pressure in the subsea hydraulic manifold rises back to its nominal level (typically, for example, 1500 psi). The BOP function is generally considered completed when the pressure gauge at the surface indicates that the subsea manifold pressure has returned to its nominal value.
In deep water, the pressure gauge on the surface may typically respond only very slowly to changes in the subsea manifold pressure; for example, the indicated pressure on the surface pressure gauge may return to the nominal manifold pressure between about 10 and 20 seconds after the selected BOP function has been actuated, which is a high percentage of the allowable BOP closing time.
Accordingly, there exists a need for a hydraulic control system for a deepwater subsea BOP stack that gives an accurate, more-rapid indication of the actuation of a selected BOP function, without depending on unreliable electrical signals such as are employed in electro-hydraulic control systems.